Acoustic light deflection cells

ABSTRACT

The efficiency and bandwidth of an acoustic deflection cell are increased by using two transducers tuned to adjacent frequency bands and buffer member for causing the acoustic waves from the two transducers to propagate in different directions that are separately optimized. A frequency-sensitive switch connects an AC source to only one of the two transducers. The buffer member is designed to minimize or eliminate light deflection by spurious reflected acoustic components.

United States I [72] lnventors Douglas A. Pinnow Berkeley Heights; Samuel R. Williamson, lrvlngton, both of NJ.

[21] Apple No. 805,189

[22] Filed Mar. 7,1969

[45] Patented Oct. 19, 1971 [73] Assignee Bell Telephone Laboratories, Incorporated Murray Hill, Berkeley Heights, NJ.

[54] ACOUSTIC LIGHT DEFLECTION CELLS 6 Claims, 2 Drawing Figs.

[52] U.S. Cl. 350/161 [51] Int. Cl G02l 1/28 [50] FieldolSearch 350/161, 160

[56) References Cited UNITED STATES PATENTS 3,419,322 12/1968 Adler 350/161 Primary Examiner-Ronald L. Wibert Assistant Examiner-J. Rothenberg Attorneys-R. J. Guenther and Arthur J. Torsiglieri ABSTRACT: The efficiency and bandwidth of an acoustic deflection cell are increased by using two transducers tuned to adjacent frequency bands and buffer member for causing the acoustic waves from the two transducers to propagate in different directions that are separately optimized. A frequencysensitive switch connects an AC source to only one of the two transducers. The buffer member is designed to minimize or eliminate light deflection by spurious reflected acoustic components.

TARGET F VARIABLE rREQUENCY souact FiiEQu ENEF 22 SENSITI E PATENTEUom 19 Ian 3.614.204 SHEEIIUF'Z F/G./ H

TARGET z r l2 I LIGHT 1 25 SOURCE I FF I6 l I l7,-

VARIABLE FREQUENCY SOURCE 1 c FREQUENCY SENSITIVE r22 SWITCH zuwgm A TTORNEV PATENTEuncr 191971 3,614,204

sum 2 0L2 BACKGROUND OF THE INVENTION This invention relates to apparatus in which acoustic waves, or sound waves, are used to deflect light. The terms light and sound, as used herein, are intended merely to denote the different mechanisms of propagating energy associated with these words. For example, light" is intended to mean wave energy at frequencies above and below as well as within the visible light spectrum, and sound" refers not only to audible acoustic energy, but also to acoustic energy at extremely short wavelengths including microwave frequencies.

Known acoustic light deflection cells typically comprise an acoustic wave-propagating medium such as water, a resonant transducer for converting electrical frequencies to sound frequencies, and a buffer member between the wavepropagating medium and the transducer. The buffer member is used to reduce the effects of mechanical impedance mismatch between the transducer and the propagating medium and, in cases such as where water is used, to support the propagating medium and provide a transmission path from the transducer to the medium.

The light beam to be diffracted is directed through the wave-propagating medium at a slight angle with respect to wave fronts of the beam of acoustic energy. The light beam is then deflected by the acoustic waves as a function of acoustic wave frequency in accordance with the principles of Drag diffraction. The acoustic wave frequency, and hence the angle of light beam deflection, is controlled by controlling the frequency of electrical energy supplied to the transducer.

The extend of acoustic wave frequency deviation, or deflection cell bandwidth, and thus the attainable light deflection deviation, is limited by several factors: First, the transducer is a resonant piezoelectric member capable of converting electrical operations to acoustic waves only over a limited bandwidth. Secondly, if the angle of incidence of the light beam is constant, maximum efficiency deflection will occur at only one acoustic frequency; as the acoustic frequency changes, the direction of light beam deflection changes as mentioned before, but deflection efficiency drops.

The buffer member normally has a different acoustic impedance than the wave-propagating medium, which necessarily results in a partially reflecting interface between the buffer member and the propagating medium. Reflected acoustic components from the buffer member may therefore enter the wave-propagating medium to give spurious deflection of the light; the acoustic beam may also be deleteriously reflected by the end of deflection cell opposite the buffer member.

Considerable effort has been made to use two orthogonally disposed cells to give the vertical and horizontal deflection required for television raster scanning. Such arrangements may also be used for accessing elements, such as individual holograms, of an optical memory system. It can be appreciated that limited angular deflection and spurious deflection can seriously limit these uses. To perhaps a smaller extend, they also limit cell use as a light beam modulator.

The paper A television Display Using Acoustic Deflection and Modulation of Coherent Light" by A. Korpel et al., Proceedings of the IEEE, Vol. 54, No. 10, Oct. 1966, pages l429-l437, and the patent of Adler, U.S. Pat..No. 3,419,322, issued Dec. 31, 1968, both described techniques for adjusting or steering" the acoustic beam as a function of frequency to give more efficient deflection. A cursory study of these references shows that the techniques proposed substantially complicate the structure and operation of deflection cells.

SUMMARY OF THE INVENTION We have found that the acoustic frequency range over which an acoustic deflection cell can be operated, and thus the attainable range of light deflection angles, can be increased by using two or more transducers on the cell, each being tuned to adjacent frequency hands. If, for example, two transducers are used, one may have a center resonant frequency of f} and bandwidth M, while the second transducer has a center frequency f; and a bandwidth 4f, having a lower frequency that approximately corresponds to the uppermost frequency of A11. The variable frequency AC source that controls light deflection is connected to one of the two transducers through a frequency-sensitive switch that directs electrical frequencies within bandwidth Af, to the first transducer and frequencies within the band Af, to the second transducer. Thus, the two transducers together have a bandwidth (Af,+Af, and a light beam can be deflected over an angle approximately twice as large as that determined by the bandwidth of a single transducer.

The two transducers may be located side by side on the buffer member so that they project acoustic beams along parallel paths. In accordance with another feature of the invention, the interface between the buffer member and the acoustic wave-propagating medium that extends through one acoustic beam path is arranged at an angle with respect to the interface that extends through the other acoustic beam path. This results in one acoustic beam being projected through the wave-propagating medium at a different angle than the other beam, and these angles are adjusted to give optimum compliance with Bragg condition for light deflection; that is, the direction of the acoustic beam from the first transducer is arranged to optimize deflection at frequency f,, and the direction of the acoustic beam from the second transducer is arranged to give maximum efficiency deflection at frequency The device as described thus far could be made with at least one interface between the buffer member and the acoustic wave-propagating medium being parallel with the plane of the corresponding transducer; in fact, this is true of conventional deflection cells. In accordance with another feature of the invention, the effects of spurious acoustic wave components reflected from the interface between the buffer member and the propagating medium are reduced or eliminated by making that interface slope at an angle with respect to the interface between the transducer and the buffer member. It will be shown later, with an appropriate angle of slope, spurious reflected components are projected into the propagating medium at an angle inappropriate for light beam deflection, thereby rendering them harmless.

These and other objects, features, and advantages of the invention will be better understood from a consideration of the following detailed description taken in conjunction with the following drawing.

DRAWING DESCRIPTION FIG. 1 is a schematic view of an illustrative embodiment of the invention; and

FIG. 2 is a schematic view of part of the deflection cell of FIG. 1 illustrating the interaction of acoustic energy and light energy in the deflection cell.

DETAILED DESCRIPTION Referring now to FIG. I, there is shown schematically a deflection cell 11 for deflecting a light beam 12 generated by a light source 13, which may preferably be a laser. The deflection cell 11 comprises an acoustic wave-propagating medium 15, a buffer member 16, a first transducer 17, a second transducer l8 and an upper member 19. A variable frequency source of alternating current electrical energy 21 is connected to either the first transducer 17 or the second transducer 18 by a frequency-sensitive switch 22. The aforementioned Korpel et al. paper teaches that, it an acoustic deflection cell, the transducer may be made of lead zirconate, the buffer member of glass, and the acoustic wave-propagating medium of water.

The purpou of the deflection cell 11, like that of similar apparatus of the prior art, is to deflect light beam 12, in accordance with the frequency of source 21, to cause it to impinge on a selected location of a target 23. As is known, two deflection cells may be used to give respectively the horizontal vertical deflection required for either raster scanning or memory accessing. A single acoustic cell may also be used to modulate the light beam, in which case the target may be a photodetector with an aperture for masking the photodetector with an aperture for masking the photodetector as a function of light beam deflection.

Water is probably the most widely used material for the propagating medium of deflection cells, although it has recently be found that crystalline a iodic acid (a H is for many purposes a more suitable material. The transducers l7 and 18 are preferably crystals of an appropriate piezoelectric material such as lithium niobate.

The first transducer 17 is mechanically resonant over a bandwidth Af, with a center frequency f,, and transducer 18 has a resonant frequency bandwidth Af, with a center frequency f, The transducers are designed such that the uppermost frequency within frequency band Af, is approximately equal to the lowermost frequency of band Af,; the larger physical size of transducer 17 is intended to connote a lower resonant frequency than that of transducer 18. The switch 22 may include a frequency responsive filter which actuates a diode to connect the source 21 to transducer 18 when the source frequency is within the range Af,, or it may take numerous other forms.

The light beam deflection can of course be controlled by either manually or automatically controlling the output frequency of source 21. When the output frequency is within range Af,, transducer 17 generates a beam of acoustic energy that deflects the light beam, but when the frequency reaches the range Af, the source 21 is automatically connected to transducer 18 and the light beam is then deflected by an acoustic beam generated by transducer 18. Hence, the deflection cell has an acoustic frequency bandwidth of substantially Af, +Af,, rather than a bandwidth limited by the resonant frequency band of a single transducer.

Referring to FIG. 2, when transducer 17 is excited it generates a beam of acoustic energy 25 having acoustic wave fronts 26. Since the wave-propagating medium and the buffer member 16 are normally made of materials having different mechanical impedances, or velocities of sound propagation, the acoustic beam 25 is refracted at the interface 27 of these two materials. As is known. the deflection cell is oriented with respect to light beam 12 such that the light beam impinges the acoustic wave fronts 26 at an appropriate angle to fulfill the conditions for Bragg diffraction at the center frequency f, of the acoustic beam. The light beam 12 is deflected by the wave fronts along an angle I given approximately by l ==AA l. where A is the wavelength of the light beam in the acousticpropagating medium and A is the acoustic wavelength of acoustic beam 25. Hence, as the acoustic frequency changes, the angle D through which the light beam 12 is deflected changes. The angle 6 for satisfying the Bragg condition is defined as sin 0,=%(A/A,) =95 (A/A,,)f, 2.

where A, is the wavelength of the acoustic beam at frequency f, and V, is the acoustic wave velocity in the propagating medium 15.

As is known, maximum efficiency light deflection occurs only at the frequency f, that satisfies the Bragg condition of equation (2). As the acoustic wave frequency deviates from f,, the efficiency of light beam deflection drops. it is therefore important that the angle 0, at which light beam 12 intersects he wave fronts of acoustic energy generated by transducer 18 different than the angle 0, at which it intersects the lower quency waves from transducer 17. To give optimum comphance with the Bragg condition and maximum efficiency deflection, the undeflected light beam 12 should be incident on wave fronts originating at transducer 18 at an angle 0, given by sin 0,--'(A/A,) =%(A/Am) f, 3.

Referring again to FIG. 1, equation (2) is met by locating the interface portion 27 in the path of acoustic beam 25 to refract the acoustic beam so that it crosses the path of the light beam at the proper angle. Likewise the interface portion 27' in the.

From equation (4). and from Snells Law, which says that refraction at an interface is proportional to the propagation velocity in the two media, it can be shown that the difference of angles 8, 8, is given by where V, is acoustic velocity in buffer medium 16. Compliance with equations (2) and (5) gives the proper direction to acoustic beam path 25' for compliance with equation (3).

Notice that equation (5) does not stipulate any absolute angles 8, and 8,, and in fact 8, could be zero. lf it were, however, a small component of acoustic energy from transducer 17 would be reflected from interface 27, reflected from transducer l7, and be projected into the propagating medium 15 such as to'harmfully interact with light beam 12.

FIG. 2 illustrates how the orientation of interface 27 at the angle 8, with respect to the planar transducer 17 reduces or eliminates the effects of such reflected components. By Snell's Law, it can be shown that the main acoustic beam 25 will be projected in medium 15 at an angle is with respect to the normal to interface 27 given by u That part of the acoustic beam reflected by interface 27 follows path 25A which, by simple geometric considerations, and by equation (6), emerges from interface 27 at an angle 32 with respect to the normal as shown Path 25A therefore extends at an angle 25 with respect to the main acoustic beam 25. It can intuitively be appreciated that if the angle 2: is designed to be sufficiently large. the reflected component will be sufficiently far removed from the Bragg condition (equation (2) to avoid spurious deflection of the light beam. Notice also that any acoustic wave component 25B reflected from upper member 19 is far removed from the Bragg condition if e is sufficiently large.

The range of angles Ad over which acoustic beam 25 deflects the light beam is given Ad =(A/A,,) Afl 7. If the angle 2c is greater than MD, the reflected acoustic com ponent cannot deflect the light beam, which leads to the condition Combining equation (6). (7), and (8) gives Vin l which leads to the requirement for 8,,

V xaf, 27; (10) When equation 10) is met, the path 25A of reflected energy will be at a sufficiently large angle with respect to the light beam 12 to substantially preclude spurious deflection.

Likewise, spurious deflection by reflected components of acoustic beam 25' of FIG. 1 will be avoided if V XAfz 52 2m (11 While the deflection cell has been described as including two transducers, it is to be understood that three, four, or more transducers can be used on a single deflection cell. Each transducer It should have a bandwidth Af, that is part of a series that defines a continuous bandwidth of the entire deflection cell. The angle 8,, that the buffer member-propagating medium interface should form is of course determined by equation (5) where the wavelength A. of the acoustic beam under consideration is used in place of A, of that equation. If so desired, the successive transducers of the deflection cell may be bonded onto the buffer plate at different angles to give different directivities to the successive acoustic beams. This construction may be preferred if an acoustic wave-propagat ing medium is used to which the transducers may be directly bonded.

Various other embodiments and modifications can be made by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In an acoustic deflection cell of the type comprising an acoustic wave-propagating medium located in the path of a light beam to be deflected, a variable frequency source of electrical energy, and means for generating acoustic waves in the medium in response to the electrical energy, the improvement wherein;

the acoustic wave-generating means comprises first and second transducers connected to the deflection cell for generating the acoustic waves; and further comprising frequency-sensitive means for connecting the source of only the first transducer when the source frequency is within a first range of values and for connecting the source to only the second transducer when the source frequency thereof is within a second range; and means for directing acoustic waves generated by the second transducer through the medium in a different direction from that of waves generated by the first transducer. 2. The improvement of claim 1 wherein: the first and second transducers are resonant devices respectively having center frequencies), and f,, frequency f being within the first range of frequency values and frequency f being within the second range of frequency values; means for directing waves generated by the first transducer through the medium at an angle 0, with respect to the light beam substantially according to the relation sin 0,=%(A/A,,,)f

medium and )t is the wavelength of light in the medium, and the means for directing the acoustic waves generated by the second transducer comprises means for directing acoustic.

wave energy of frequency f, at an angle 6, with respect to the light beam substantially according to the relation sin 0,=%(A/A )f,. 3. The improvement of claim 2 wherein: the means for directing acoustic waves generated by the first and second transducers comprises a buffer member between the acoustic wave-propagating medium and the transducers, whereby acoustic waves propagate in the buffer member prior to transmission to the acoustic wave propagating medium. 4. The improvement of claim 3 wherein: the buffer member has a different acoustic velocity of propagation than the acoustic wave-propagating medium and forms an interface with the acoustic wave-propagating medium; acoustic waves generated by the first transducer are transmitted through a first planar interface portion and acoustic waves generated by the second transducer are transmitted through a second planar interface portion; the second planar interface portion being disposed at an angle with respect 0 the first and second transducers are refracted at different angles by the first and second interface portions. 5. The improvement of claim 4 wherein: the first transducer fonns a first transducer interface with the buffer member; the second transducer fonns a second transducer interface with the buffer member; the angle 6, of the first interface portion with respect to the first transducer interface substantially conforms to the relation 5 2'TI'H X-Af 1 where V, is the velocity of sound in the buffer member, V, is the velocity of sound in the acoustic wave-propagating medium, and Af, is the frequency bandwidth of the first transducer.

6. The improvement of claim 5 wherein: the angle 8, of the second interface portion with respect to the second transducer interface substantially conforms to the relation where A is the wavelength of sound in the acoustic wavecyf, 

1. In an acoustic deflection cell of the type comprising an acoustic wave-propagating medium located in the path of a light beam to be deflected, a variable frequency source of electrical energy, and means for generating acoustic waves in the medium in response to the electrical energy, the improvement wherein: the acoustic wave-generating means comprises first and second transducers connected to the deflection cell for generating the acoustic waves; and further comprising frequency-sensitive means for connecting the source of only the first transducer when the source frequency is within a first range of values and for connecting the source to only the second transducer when the source frequency thereof is within a second range; and means for directing acoustic waves generated by the second transducer through the medium in a different direction from that of waves generated by the first transducer.
 2. The improvement of claim 1 wherein: the first and second transducers are resonant devices respectively having center frequencies f1 and f2, frequency f1 being within the first range of frequency values and frequency f2 being within the second range of frequency values; means for directing waves generated by the first transducer through the medium at an angle theta 1 with respect to the light beam substantially according to the relation sin theta 1 1/2 ( lambda / Lambda m)f1 where Vm is the acoustic wave velocity in the propagating medium and lambda is the wavelength of light in the medium, and the means for directing the acoustic waves generated by the second transducer comprises means for directing acoustic wave energy of frequency f2 at an angle theta 2 with respect to the light beam substantially according to the relation sin theta 2 1/2 ( lambda / Lambda m)f2.
 3. The improvement of claim 2 wherein: the means for directing acoustic waves generated by the first and second transducers comprises a buffer member between the acoustic wave-propagating medium and the transducers, whereby acoustic waves propagate in the buffer member prior to transmission to the acoustic wave- propagating medium.
 4. The improvement of claim 3 wherein: the buffer member has a different acoustic velocity of propagation than the acoustic wave-propagating medium and forms an interface with the acoustic wave-propagating medium; acoustic waves generated by the first transducer are transmitted through a first planar interface portion and acoustic waves generated by the second transducer are transmitted through a second planar interface portion; the second planar interface portion being disposed at an angle with respect o the first and second transducers are refracted at different angles by the first and second interface portions.
 5. The improvement of claim 4 wherein: the first transducer forms a first transducer interface with the buffer member; the second transducer forms a second transducer interface with the buffer member; the angle delta 1 of the first interface portion with respect to the first transducer interface substantially conforms to the relation where Vb is the velocity of sound in the buffer member, Vm is the velocity of sound in the acoustic wave-propagating medium, and Delta f1 is the frequency bandwidth of the first transducer.
 6. The improvement of claim 5 wherein: the angle delta 2 of the second interface portion with respect to the second transducer interface substantially conforms to the relation where Lambda 1 is the wavelength of sound in the acoustic wave-propagating medium at frequency f1 and Lambda 2 is the wavelength of sound in the acoustic wave-propagating medium at frequency f2. 